Optimized scintillator and pixilated photodiode detector array for multi-slice CT x-ray detector using backside illumination

ABSTRACT

A photodiode detector array includes a layer of intrinsic semiconductor material having a first doped layer on a first surface of a first conductivity type and an array of photodiodes having respective doped regions on a second surface of an opposite conductivity type. Electrical contacts on the second surface respectively contact the doped regions and convey electrical signal therefrom. Conductors extend from the electrical contacts to convey the electrical signals to output terminals of the array. A scintillator is optically coupled to the layer of intrinsic semiconductor material at the first surface thereof and can be pixelated, with individual scintillator elements aligned with and corresponding to the doped regions of the photodiode. The photodiode detector array can be mounted to a rigid printed wiring board or to a flat bottom wall surface of the scintillator.

BACKGROUND OF THE INVENTION

[0001] This invention relates to a multi-slice computed tomography (CT)x-ray detector and, more particularly, to a photodiode array activatedby backside illumination affording access for electrical connections toindividual photodiodes at the opposite, usual front side of the array,while maintaining desired values of the modulation transfer function(MTF) of the detector, enabling the assembly of large two-dimensionaldetector arrays.

[0002] Radiation imaging systems employing such detectors are widelyused for medical and industrial purposes, such as for x-ray computedtomography (CT); a typical detector may comprise an array ofsemiconductor photodiodes, or photodiodes, used to detect light or otherionizing radiation, having attached scintillators. To increase imagequality and speed of such detectors, a large number of individual pixelsis required. Present technology uses on the order of 1000 to 4000individual pixels with a respective amplifier per pixel. Someimplementations (e.g., GE LIGHTSPEED™ scanners) have configurabledetectors wherein plural, respective signal currents from multipleindividual photodiodes can be combined for further processing in asingle amplifier channel. This arrangement permits the detection areafor an individual pixel to be varied, using externally controlledelectrical switches. However, as the number of individual amplifierchannels and respective pixels is further increased to a desirablenumber, e.g., 18 12000 or more, providing all necessary electricalconnections becomes complex and cumbersome.

[0003] Present technology uses a single amplifier per photodiode, andthus per pixel, since this affords high data rates and high signalquality. Moreover, present technology provides connections from thephotodiodes to the respective amplifiers at the edges of the detectorarrays, using a flexible interconnector structure, such as a flexiblecircuit board (“flex”) that brings all of the amplifier connections toedges of the photodiode arrays. However, as the number of amplifierchannels increases, the density of the interconnector structuresincreases to an unattractively high level from the standpoint ofcomplexity, ease of fabrication, and performance. This structure alsoplaces some practical fabrication limitations on expanding the area ofthe array.

[0004] It is desirable to provide an imaging device that permitsincreasing the density of photodiode detection elements in a photodiodearray chip and, as well, the total number of photodiodes and the area ofthe array.

BRIEF SUMMARY OF THE INVENTION

[0005] In one representative embodiment, a photodiode detector array isprovided that includes a layer of intrinsic semiconductor materialhaving first and second opposite main surfaces, a first doped layer atthe first surface of a first conductivity type, and an array ofphotodiodes on the second main surface comprising respective dopedregions of a second conductivity type. The detector array furtherincludes electrical contacts coupled to the second main surface,respectively contacting the doped regions and adapted to conveyelectrical signals therefrom. Conductors are coupled to the electricalcontacts, and a scintillator is optically coupled to the first mainsurface of the intrinsic semiconductor material. It should beappreciated that intrinsic semiconductor material comprises a lightlydoped semiconductor of the first conductivity type, and the use of theterm intrinsic describes such a lightly doped semiconductor.

[0006] In another representative embodiment, a method of fabricating aphotodiode detector array for use in an x-ray detector is provided. Themethod comprises the steps of forming a layer of intrinsic semiconductormaterial on a substrate. The layer of intrinsic semiconductor comprisesa first surface and a second surface where the first surface ispositioned opposite from the second surface. A first doped layer isprovided and positioned at the first surface. The first doped layercomprises a first conductivity type. A plurality of second doped regionsis provided and positioned at the second surface. The second dopedregion comprises a second conductivity type. The first conductivity typeis opposite to the second conductivity type where the plurality ofsecond doped regions detects radiation incident on the first surface andoutputs electrical signals corresponding to the incident radiation. Eachof a plurality of electrical contacts is connected to a different one ofthe plurality of second doped regions. The plurality of electricalcontacts extends along the second surface. A first plurality ofconductive electrode pads is located on a first board surface of aprinted wiring board. Each of the first plurality of conductiveelectrode pads is aligned with a different one of the plurality ofsecond doped regions, and the printed wiring board is positionedproximate to the second surface. A second plurality of conductiveelectrode pads is located on a second board surface of the printedwiring board. The second board surface is located opposite from firstboard surface, and each of the second plurality of conductive electrodepads is connected to a different one of the first plurality ofconductive electrode pads. The layer of intrinsic semiconductor materialis positioned with the second surface connected to the first boardsurface, and each of the plurality of electrical contacts is alignedwith a different one of the first plurality of conductive electrodepads. A conductive epoxy is applied between each of the plurality ofelectrical contacts aligned with the first plurality of conductiveelectrode pads, and each of the plurality of electrical contacts iselectrically connected to a different one of the first plurality ofconductive electrode pads.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a schematic diagram of a multi-slice CT x-ray detectoremploying a photodiode array chip using backside illumination inaccordance with the present invention.

[0008]FIG. 2 illustrates a portion of the photodiode array chip of FIG.1, in an enlarged scale in the lateral (Z) direction but a decreasedscale in the vertical (X) direction relative to the structure of FIG. 1.

[0009]FIG. 3 is a fragmentary elevational cross-sectional view of amulti-slice CT x-ray detector in accordance with a representativeembodiment of the present invention, illustrating a arrangement ofelectric connections to the photodiode array chip.

[0010]FIG. 4 is schematic diagram of a structure in accordance with onerepresentative embodiment of the present invention for analyzing theeffect of light being incident on the opposite side of a photodiodearray chip, relative to the norm.

[0011]FIG. 5 is a schematic and cross sectional elevational view of ascintillator, useable with each of the multi-slice CT X-ray detectorembodiments of the present invention, having a structure which restrictsthe level of cross talk between photodiodes of a photodiode array chipcoupled thereto, permitting improved characteristics of enlarged arrays.

[0012]FIG. 6 is a plot of fractional lost signal values relative to theratio of scintillator exit diameter to pixel pitch, for variousdifferent thicknesses of an intrinsic layer.

[0013]FIGS. 7 and 8 are illustrations of alternative embodiments ofscintillator structures for use in the combination of FIG. 5.

[0014]FIG. 9 is a fragmentary elevational cross-sectional view of amulti-slice CT x-ray detector in accordance with a second embodiment ofthe invention employing edge connectors.

[0015]FIG. 10 illustrates an arrangement of multiple detector modulestiled in the X-direction only and employing edge connectors as in thesecond embodiment of the invention shown in FIG. 9.

[0016]FIG. 11 is a perspective and exploded view of a multi-slice CTx-ray detector in accordance with a third embodiment of the invention,having an alternative rear connector structure.

DETAILED DESCRIPTION OF THE INVENTION

[0017] As shown in FIG. 1, a multi-slice CT x-ray detector 100 comprisesan i-bulk (intrinsic) semiconductor layer 102 formed on substrate 112.While disclosed specifically in relation to x-ray detection, thestructure described herein is suitable for detection of various forms ofhigh energy ionizing radiation, including, for example, gamma rays, highenergy electron (beta) rays or high energy charged particles (as areencountered in nuclear physics and space telescopes). Therefore, itshould be appreciated that the present invention is not limited to thespecific disclosed embodiment of x-ray detection.

[0018] The photodiode array chip 101 comprises an i-bulk (intrinsic)layer 102 having an n+ doped layer 104 at the upper main surface, asseen in FIG. 1, and p+ doped regions 106 at the opposite, lower mainsurface of the i-bulk layer 102, supported on a substrate 112. It is tobe understood that additional layers, e.g., an oxide layer, can beformed additionally on either or both of the aforesaid upper and lowermain surfaces (the latter terms being used, as well, to refergenerically to the outermost surfaces of the layer 102). Lightrepresented by arrows 119 and emitted from a scintillator 114, and moreparticularly from individual scintillator elements 116 thereof havingrespective exit windows 117 respectively associated, and aligned (thatis, disposed so that rays emanating from window 117 and perpendicular toupper surface of layer 102 will impinge on the bordering p+ region) withcorresponding doped regions 106 representing corresponding photodiodes,is incident on and enters the photodiode array chip 101 from the upper,n+doped layer side.

[0019] It is noted that the arrangement, shown in FIG. 1, is “inverted”with respect to the conventional arrangement of a radiation detectingarray in which the heavily doped regions corresponding to the pixels(e.g., as would correspond to regions 106 in FIG. 1) are disposedadjacent to the surface of the chip 101 through which radiation entersthe array.

[0020] In FIG. 2, a portion of the detector 100 in FIG. 1, is drawn toapproximate, scaled relative dimensions for a chip thickness (W) ofabout 100 microns and an array pitch (Pa) (length of pixel (P) plus thelength of the gap (G) between adjacent edges of the doped regions 106)of about 1000 microns. The array pitch (Pa) is also termed pixel pitch.In one embodiment, typical dimensions of components of the structure inFIGS. 1 and 2 are provided in the following Table 1 in which thedimensions of W, G and P are in the X-direction as shown in FIG. 1.TABLE 1 Dimensions W(Intrinsic silicon (Si) layer 102 ˜100 micronsthickness-i.e., chip thickness) Wn(N + layer 104 depth) ˜0.5 microns P(p + region lateral dimension) ˜762 microns G (gap between p + regions)˜262 microns Ps (scintillator element lateral dimension) ˜910 microns

[0021] Respective, individual connections to the (p+) doped regions 106are schematically indicated by a conductor 108 connected to theleft-most p+ region 106 in FIG. 1. As shown in FIG. 3, one embodiment ofelectrical connections of a CT x-ray detector 100 is provided.

[0022] The photodiode array chip 101, due to its inverted configurationrelative to the conventional photodiode array chip, has a potentialproblem of not affording adequate spatial resolution, known as the“modulation transfer function” (MTF) which is a measure of the spatialresolution, or cross-talk, in a pixelated detector. More particularly, atypical hole generation depth (i.e., in the intrinsic-bulk layer 102,below the junction with the n+ layer 104) is about 2 microns for thedominant 610 nm wavelength light from the scintillator 114. The invertedorientation of the structure permits a lateral spread of holes withinthe intrinsic layer 102 before collection of the holes by the p+photodiodes on the other (i.e., the bottom, or lower) side of the layer102. As used herein, “upper,” “lower,” “top,” “bottom” and the like areused to delineate relative location of components in the drawings anddoes not imply any operational limitations or orientations.

[0023] With reference to FIGS. 1 and 2, it can be seen that, given therelative distances involved, light incident on chip 101 p+ doped region106-1 of one photodiode will be unlikely to produce a signal in the p+doped region 106-2 of the adjacent photodiode. Lateral diffusion ofholes from the point of incidence on chip 101 would be approximately ofthe same magnitude as the thickness W_(n) of the n+ doped layer 104.Hence, for the example dimensions specified in Table 1, the MTF for theinverted structure of the invention is essentially unchanged compared toconventional detector design. Based on these analyses, it is possible toincrease the intrinsic layer thickness to about 200 microns or morebefore the MTF begins to degrade, which is relevant to fabricationconsiderations, as later discussed.

[0024] In FIG. 3, an n+ doped region 107 is disposed in the photodiodearray chip 101, between two adjacent p+ doped regions 106-1 and 106-2. Apatterned dielectric layer 130 has mounted therein electrical contacts132-1 and 132-2 disposed in electrical contact with the p+ doped regions106-1 and 106-2, respectively, and a further electrical contact 134disposed in electrical contact the n+ doped region 107. Doped region 107and its associated electrical contact 134 provide a cathode contactwhich may suffice for all photodiodes of the photodiode array chip 101.Alternatively, two or more such n+ doped regions 107 and associatedcontacts 134 may be incorporated in the photodiode array chip 101. Inessence, doped zone 107 provides an n+ zone contact through intrinsiclayer 102 to the n+ layer 104 which effectively serves as a ground planefor the photodiode array chip 101.

[0025] A printed wiring board (PWB) 120, or similar substrate, has large(i.e., corresponding in lateral dimensions to overlying components towhich contact is to be made) metal pads 122 and 124 on a first, uppersurface 121 of the PWB 120. Conductive elements 133-1, 133-2 and 135respectively connect the metal pads 122-1, 122-2 and 124 to thecorresponding doped regions 106-1, 106-2 and 107. Conductive elements133-1, 133-2 and 135 may be formed of patterned conductive paste orsolder, or by a uniform, continuous layer of a conductive adhesive withvertically (i.e., Y direction, in FIG. 3) conductive characteristics(i.e., which affords no lateral, or Z-direction, of conduction).Respective electrical connections 123 and 125 to the pads 122 and 124are routed through the PWB 120 to respective metal pads 126 and 128 on asecond, lower surface 127 of the PWB 120. The metal pads 126 and 128 canthen be interfaced to readout amplifiers using standard connectortechniques and devices, such as metal-to-metal pressure connectors oranisotropic elastomeric films.

[0026] Because the intrinsic silicon photodiode layer 102 of thephotodiode is relatively thin, having a thickness between about 100microns and about 200 microns (but which could be made thicker, e.g.,400 microns, for a medical CT, albeit with some degraded image quality),a support structure is required for the layer 102 on at least one of itssurfaces. Wiring board 120, as shown in FIG. 3, is a suitable supportstructure. In conventional photodiode array structures the siliconthickness in the chip is ˜500 microns, which provides mechanicalstrength during processing.

[0027] In another embodiment, the photodiode array chip 101 can bebonded to, and thus supported by, the scintillator 114 by an opticalcoupling adhesive layer 105. The PWB 120 (or, alternatively, a flexconnector) is then attached to the dielectric layer 130 on the lowersurface of the intrinsic layer 102, using a conductive epoxy, tocomplete electrical connections between the metal pads 122-1, 122-2 and124 and the electrical contacts 132-1, 132-2 and 134, respectively. Thearrangement in which the scintillator block 114 provides the structuralsupport reduces mechanical requirements on the PWB 120 and serves toreduce costs.

[0028] In FIG. 4, a one dimensional (1-D) detector, representing asingle photodiode element of the array 101, is provided for an analysisof the effect of the light being incident on the photodiode array fromthe opposite side of the detector, relative to a conventionalconfiguration. The analysis is performed by solving the minority carrierdiffusion equation for carriers generated at a variable depth Z in theintrinsic layer 102, as shown in FIG. 4. The probability (P_(loss)) thata minority carrier (hole) generated at a depth Z will not contribute tothe photocurrent (loss fraction) is:

P _(loss)(Z,W,Lp)=1−cosh(z/Lp)/cosh(W/Lp)  (1)

[0029] wherein Lp is the minority carrier diffusion length in the Nlayer. For a high quality photodiode process, Lp˜1000 microns or larger.As provided hereinabove and shown in FIG. 1, 4, 5 and 7-8, W representsthe chip thickness.

[0030] The polarities within the photodiode array of the presentinvention, as shown in the preceding FIGS. 1 to 4, are not limiting andthe opposite respective polarities may be employed in the alternative.In such an alternative design, light would be incident from the p+ sideand progress through a lightly doped layer p-type silicon layer to n+pixel photodiodes.

[0031] In FIG. 5, a scintillator 140 is provided that is useable withthe detector arrays 101, and the scintillator 140 further has structurethat restricts the level of cross talk between the photodiode p+ dopedregions 106. More particularly, scintillator 140 is shown assembled witha multi-slice CT x-ray detector 100 having the n+ incident geometry(that is, light is incident on the array through n+ layer 104) as shownin FIG. 1. Scintillator 140 has plural scintillator elements 140-1 and140-2, 140-3 of a width Ps and arranged at a corresponding pitchcorresponding to the diode array pitch (Pa), and aligned with, thephotodiodes of the photodiode chip array 101 as defined by the p+regions 106-1, 106-2, 106-3, etc. The walls of each scintillatorelement, are formed of an optical reflector material and the bottom wallof each includes an optically transparent exit window 146 of a widthdimension (Po) that is smaller than the lateral (Z) dimension “Ps” ofthe respective scintillator elements. The aperture width (Po) of thescintillator elements 104-1, 104-2, 140-3 is also termed scintillatorexit diameter. An illustrative x-ray photon 142 is shown at a point ofabsorption in scintillator pixel 140-2 where it is converted to multiplelight beams, or paths of photons, of which there is shown anillustrative light beam 144 which undergoes a limited number of multiplereflections within the interior of the scintillator pixel 140-2 beforeexiting through the exit window 146. The light beams are not required tobe specular and, typically, the light beams instead are diffused,consistent with light photons being emitted in all directions.Hereinafter, however, illustration of an individual and specular lightbeam is adopted for convenience of illustration and discussion. Thescintillator 140 is adhered to the detector array 100 by a layer 148 oftransparent optical coupling material through which the light beampasses and then is incident on the n+ layer 104 of the array 101, asdiscussed in connection with FIG. 1 hereinabove.

[0032] Cross talk (MTF degradation) is most likely when light isincident near the edges of the p+ regions 106 or in the gap between thep+ regions. However, due to the aperture width (Po) of the window 146,which restricts the exiting light substantially to the central portionsof the p+ regions 106 corresponding to the respective scintillatorelements 140, MTF degradation is reduced, enabling higher density in thephotodiode arrays (that is, the gap (G) between respective P+ regions106 can be reduced) and thus provides improved resolution of x-rayimages. The embodiment shown in FIG. 5 accordingly allows the thickness(W) of the silicon wafer 101 or the array pitch (Pa) of the photodiodesto be reduced below those values possible with the embodiment of FIG. 1employing a conventional (non-apertured) scintillator 114. In otherwords, reducing the array pitch (Pa) improves the resolution of thedisplay but increases the probability of cross talk; on the other hand,reducing the sized aperture width (Po) reduces the probability of crosstalk. As the aperture width (Po) is reduced and all other parameters areheld constant, the amount of cross talk is reduced, since the carriersgenerated in the silicon would have to diffuse a greater lateraldistance (Z-direction) to produce cross talk.

[0033] In FIG. 6, a plot of the fraction lost signal (i.e., decimalfaction values) of the signal charge, or carriers, which are notcollected by a given, intended photodiode but which are lost to one ormore adjacent (i.e., nearest neighbors) photodiodes (presented on theordinate) is provided with respect to differing scintillator elementexit diameters (Po)/Ps, (presented on the abscissa) and for each ofplural ratios P_(I1) through P_(I6) of the specified ratio of valuesdesignated in FIG. 6 to the right of the plot and thereby indicating thetotal lost signal (or carrier) fraction for each of the values P_(I1)through P_(I6).

[0034] As further shown in FIGS. 7, the scintillator 150 includes aplurality of scintillator elements 150-1, 150-3 and 150-3. Each of theplurality of scintillator elements 150-1, 150-3 and 150-3 include acommonly oriented top wall 151 and bottom wall 153. In one embodiment,the top wall 151 and the bottom wall 153 have a light reflectivematerial disposed thereon. Further, each of the plurality ofscintillator elements 150-1, 150-3 and 150-3 include a first sidewall155 and a second sidewall 157 that extend from the top wall 151 to thebottom wall 153. In one embodiment, the first sidewall 155 and thesecond sidewall 157 have a light reflective material disposed thereon. Awindow 156 is positioned in the bottom wall 153, and the window 156 hasa lateral dimension (width) that is less than a lateral dimension(width) of the bottom wall 156. In FIG. 7, each scintillator element150-1, 150-2 and 150-3 includes interior bottom walls 152 (also termedportions of the first sidewall 155 and second sidewall 167) that areadjacent the bottom wall 153. The interior bottom walls 152 slope, at apredetermined angle, inwardly toward the window 156 and extenddiagonally between adjacent surfaces of the first sidewall 155 andbottom wall 153 and between adjacent interior surfaces of the secondsidewall 157 and the bottom wall 153. In one embodiment as shown in FIG.7, the window 156 is composed of a layer of transparent opticalmaterial. Further, the first sidewall 155 and the second sidewall 157 ofthe scintillator elements 150-1, 150-2 and 150-3 slope inwardly toward,and contact and surround, a periphery of the window 156, such that aportion of the bottom wall 153 contacting and surrounding a windowperiphery comprises substantially a common thickness with that of thewindow 156.

[0035] Also as shown in FIG. 8, the scintillator 160 includes aplurality of scintillator elements 160-1, 160-3 and 160-3. Each of theplurality of scintillator elements 160-1, 160-3 and 160-3 include acommonly oriented top wall 161 and bottom wall 163. In one embodiment,the top wall 161 and the bottom wall 163 have a light reflectivematerial disposed thereon. Further, each of the plurality ofscintillator elements 160-1, 160-3 and 160-3 include a first sidewall165 and a second sidewall 167 that extend from the top wall 161 to thebottom wall 163. In one embodiment, the first sidewall 165 and thesecond sidewall 167 have a light reflective material disposed thereon. Awindow 166 is positioned in the bottom wall 163, and the window 166 hasa lateral dimension (width) that is less than a lateral dimension(width) of the bottom wall 166. In FIG. 8, each scintillator element160-1, 160-2 and 160-3 includes interior bottom walls 162 (also termedportions of the first sidewall 165 and second sidewall 167) that areadjacent the bottom wall 163. The interior bottom walls 162 slope, at apredetermined angle, inwardly toward the window 166 and extenddiagonally between adjacent surfaces of the first sidewall 165 andbottom wall 163 and between adjacent interior surfaces of the secondsidewall 167 and the bottom wall 163. In one embodiment as shown in FIG.8, the first sidewall 165 and the second sidewall 167 of thescintillator elements 160-1, 160-2 and 160-3 slope, at a predeterminedangle, inwardly toward, and define, a perimeter of the window 163 in aplane common with an exterior surface of the bottom wall 163.

[0036] Further, in FIGS. 7 and 8, scintillators 150 and 160,respectively, have a reduced size exit windows 156 and 166,respectively, affording the same advantages as scintillator 140 of FIG.5, and having additional features enhancing those advantages.Particularly, the sloped interior bottom walls 152 and 162,respectively, of the scintillators 150 and 160 provide substantiallydiffuse reflection of the light beam 154 (as shown in of FIG. 7) forminimizing the number of reflections while directing the light beamthrough the exit windows 156 and 166, respectively. In one embodiment,the side wall slope angle are between twenty (20) and eighty (80)degrees in relation to the plane of the exit window 156 or 166 or asidewall 155, 157 or 165, 167. The scintillator exit window 156 can beformed by a transparent optical coupler material, as in the case of theFIG. 7 scintillator 140. The scintillator 150 of FIG. 8 will achievesubstantially the same reduction in the number of reflections of thelight beam as occur in the case of the scintillator 150 of FIG. 7.However, the optical reflector material in the scintillator 160 of FIG.8 is flush with the lower bottom surface of the scintillator 160,enabling a more simplified process of machining and polishing theoptical reflector relative to the structure of the scintillator 150 ofFIG. 7. The scintillators 150 and 160 are coupled to a detector 100, asin the case of FIG. 5, by respective layers 158 and 168 of transparentoptical coupler material.

[0037] However, as the array pitch (Pa) decreases or the chip thicknessW increases, the amount of cross talk, caused by light incident near thedoped regions 106 or in the gap (G) between doped regions 106 willincrease. In one embodiment, total cross talk is defined by anexperiment where radiation is incident on only one pixel. In anotherembodiment, total cross talk is the ratio of the sum of the signals onall adjacent pixels to the sum total of the signals on all pixels,including an illuminated pixel. By confining the light emitted from thescintillator to a region centered on the respective doped regions 106,the amount of cross talk is reduced. For an array pitch on the order of˜1 mm, the desirable value of chip width to provide acceptably lowcross-talk with a conventional scintillator is less than 150 microns,which presents processing difficulties. With a scintillator with thewindow structure as described above, a chip thickness of W˜500 micronsor more may be employed with an acceptable amount of cross talk. In oneembodiment, an acceptable level of cross talk is less than about fifty(50) percent. In another embodiment, an acceptable level of cross talkis less than about ten (10) percent. In even another embodiment,standard silicon wafer thickness is 300 to 600 microns.

[0038] In FIG. 9, another embodiment is provided for use with two ormore CT x-ray detectors 100 that are not tiled in the z-direction. Inthis embodiment, connections on the back of PWB 120 are not requiredand, instead, the PWB 120 may be larger in the z-direction than thephotodiode chip array 101 and the connections may be made at one or morelaterally (Z) projecting edges of the photodiode chip array 101. In FIG.9, elements that are similar to those of FIG. 3 are identified byidentical reference numerals. However, in FIG. 9, the detector 100comprises an interconnection arrangement using a vertically conductivelayer 233, mentioned hereinabove, having vertically (Y) conductive pathsand that is formed as a uniform, continuous layer between the PWB 120and the rear, or bottom, surface of the patterned dielectric layer 130,selectively interconnecting the electrical contacts 132-1, 132-2 and 134with the pads 122-1, 122-2 and 124, respectively. As before noted, PWB120 is larger than the photodiode chip array 101 and is illustrated inFIG. 9 as extending laterally beyond, i.e., to the left of, the leftedge of the chip array 101. A surface conductor 223, formed on PWB 120,is connected at one end to metal pad 122-1 and at its other end to anedge connector 229. Metal pad 124 is connected through an alternativepath including a conductor 224, which may be a plated through-hole, andwhich extends from pad 124 to the bottom main surface of PWB 120, andthrough a conductor 225 formed thereon to a different, respective edgeconnector (not shown). The embodiment of FIG. 9 employs a non-pixilated,or uniform, scintillator 216 that uniformly radiates the photodiodes ofthe array chip 101, in contrast to the pixelated scintillator 114 of theprior embodiments. In this case, part of or all of the PWB 120 can bereplaced with a flex connector. The PWB or flex may also contain all orpart of the amplifiers and the digital-to-analog converters (“DAC”)required for encoding the charge or current from the photodiodes.

[0039]FIG. 10 is a simplified illustration of multiple multi-slice CTX-ray detector modules 100-1, 100-2, 100-3 tiered in the X direction andarranged on respective, separate PWB's 241, 242, 243 having respectivefirst edge connectors 251, 252, 253 along first commonly oriented sidesthereof and respective second edge connectors 261, 262, 263. Alongrespective, second and opposite, commonly oriented sides thereof, eachconnected by corresponding conductors 255 to corresponding electrodes onthe back surface of the detector modules 100-1, 100-2, 100-3respectively. The capability afforded by the back mounted photodiodedetector arrays of the multi-slice CT X-ray detectors 100 of theinvention facilitates the electrical connection arrangements of FIG. 11,whereby the output signals of the detector modules may be captured fromthe backside of the photodiode array and which would be prevented byconventional such connection arrangements wherein the wires must runalong the top surface of the photodiode array, between the pixels thatis a very limiting requirement because there is only a finite amount ofroom between the pixels.

[0040] In FIG. 11, a multi-slice CT X-ray detector 200 is providedhaving a photodiode chip array 201 attached to a scintillator 214, andgenerally corresponding to the detector 200 having the photodiode chiparray 201 and scintillator 214 discussed above. The PWB 220 has eightprocessing chips 230 mounted thereon, each chip 230 including dataacquisition circuitry (amplifiers, analog to digital circuits (ADC) andcontrol logic) to be electrically connected to, and processing outputsignals from, in one embodiment, for example, 64 photodiodes of thephotodiode array chip 201. In this embodiment, this configurationcorresponds to a photodiode array of 512 photodiodes, or pixels.

[0041] By digitizing the photodiode output signals, amplifying same, A/Dconverting and then multiplexing same, the 512 pixels/photodiodesoutputs are readily transmitted over an 8 bit bus flex 226 having aconnector 227 which connects to a connector 228 on the flex 220.

[0042] The foregoing discussion of the invention has been presented forpurposes of illustration and description. Further, the description isnot intended to limit the invention to the form disclosed herein.Consequently, variations and modifications commensurate with the aboveteachings and with the skill and knowledge of the relevant art arewithin the scope of the present invention. The embodiment describedherein above is further intended to explain the best mode presentlyknown of practicing the invention and to enable others skilled in theart to utilize the invention as such, or in other embodiments, and withthe various modifications required by their particular application oruses of the invention. It is intended that the appended claims beconstrued to include alternative embodiments to the extent permitted bythe prior art.

What is claimed is:
 1. A photodiode detector array for use with an x-raydetector, the photodiode detector array comprising: a layer of intrinsicsemiconductor material positioned on a substrate, the layer of intrinsicsemiconductor comprising: a first surface and second surface, the secondsurface positioned proximate to the substrate wherein the first surfaceis positioned opposite from the second surface; a first doped layerpositioned at the first surface and comprising a first conductivitytype; a plurality of photodiodes positioned at the second surface, eachof the plurality of photodiodes comprising a second doped layercomprising a second conductivity type wherein the first conductivitytype is opposite to the second conductivity type; and a plurality ofelectrical contacts positioned at the second surface, each respectiveone of said plurality of electrical contacts connected to a differentone of said plurality of photodiodes; a scintillator connected to thex-ray detector and optically coupled to the first doped layer positionedat the first surface wherein radiation emanating the scintillatorimpinges on at least one of the plurality of photodiodes; and aplurality of conductors wherein each of the plurality of conductors isconnected to a different one of the plurality of electrical contacts,the plurality of conductors extending along the second surface of thelayer of intrinsic semiconductor material.
 2. The photodiode detectorarray of claim 1 wherein said first conductivity type is n+ type andsaid second conductivity type is p+ type.
 3. The photodiode detectorarray of claim 1 wherein the scintillator comprises a plurality ofscintillator elements, each respective one of the plurality ofscintillator elements aligned with a different one of the plurality ofphotodiodes.
 4. The photodiode detector array of claim 3 wherein each ofthe plurality of scintillator elements comprises: a commonly orientedtop wall and bottom wall, the top wall and bottom wall beingsubstantially parallel to the first surface and second surface andhaving a light reflective material disposed thereon; a first sidewalland a second sidewall extending from the top wall to the bottom wall,the first sidewall and the second sidewall having a light reflectivematerial disposed thereon; and a window positioned in the bottom wall,the window having a lateral dimension less than a lateral dimension ofthe bottom wall wherein the window is aligned with a respectively one ofthe plurality of photodiodes.
 5. The photodiode detector array of claim4 wherein each of the plurality of photodiodes have a lateral dimensionin a plane parallel to the first surface and the second surface, thelateral dimension of each of the plurality of photodiodes being lessthan the lateral dimension of the bottom wall of each of the pluralityof scintillator elements but greater than the lateral dimensions of thewindow of the bottom wall in each of the plurality of scintillatorelements.
 6. The photodiode detector array of claim 4 wherein portionsof interior surfaces of the first sidewall and second sidewall of eachof the plurality of scintillator elements, adjacent the bottom wallthereof, slope inwardly toward the window and extending diagonallybetween adjacent, respective interior surfaces of the first sidewall,second sidewall and bottom wall.
 7. The photodiode detector array ofclaim 6 wherein: the window in the bottom wall of each of the pluralityof scintillator elements comprises a layer of transparent opticalmaterial; and the first sidewall and the second sidewall of each of theplurality of scintillator element slope inwardly toward, and contact andsurround, a periphery of the respective window wherein a portion of thebottom wall contacting and surrounding a periphery of the window hassubstantially a common thickness with that of the window.
 8. Thephotodiode detector array of claim 6 wherein the first sidewall andsecond sidewall of each of the plurality of scintillator element slopeinwardly toward and define a perimeter of the window in a plane commonwith an exterior surface of the bottom wall thereof.
 9. The photodiodedetector array of claim 1 further comprising an optical couplingadhesive layer positioned between the scintillator and the first dopedlayer and optically coupling the scintillator to the first doped layerat the first surface.
 10. The photodiode detector array of claim 1wherein the layer of intrinsic semiconductor material comprises athickness measured between the first surface and the second surface, thethickness ranging from about 100 microns to about 200 microns.
 11. Thephotodiode detector array of claim 1 wherein the layer of intrinsicsemiconductor material comprises a thickness measured between the firstsurface and the second surface, the thickness being about 100 microns.12. An x-ray detector, comprising: a photodiode detector arraycomprising: a layer of intrinsic semiconductor material comprising afirst surface and second surface; a first doped layer positioned at thefirst surface and comprising a first conductivity type; and a pluralityof photodiodes positioned at the second surface and each of theplurality of photodiodes comprising a second doped layer comprising asecond conductivity type; a scintillator positioned at the first surfaceand comprising: a first outer surface defining a radiation input end;and a second outer surface defining a radiation output end wherein thescintillator produces light beams exiting from the output end inresponse to radiation incident on the input end, the light beams exitingfrom the output end being incident on at least one of the plurality ofphotodiodes positioned at the second surface; and a plurality ofelectrical contacts positioned at the second surface wherein each one ofthe plurality of electrical contacts being coupled to a different one ofthe plurality of photodiodes and the plurality of electrical contactsextending along the second surface to conveying electrical outputsignals of the respective photodiodes produced in response to the lightbeams incident upon the scintillator.
 13. The x-ray detector claim 12wherein each of the plurality of electrical contacts further comprises:a portion of conductive material contacting a cathode region of arespective one of the plurality of photodiodes; and a printed wiringboard comprising a metal pads on a first board surface, the metal padpositioned to contact a corresponding portion of conductive material andextending through the printed wiring board to a second board surfacewherein the second board surface is opposite from the first boardsurface.
 14. The x-ray detector of claim 12 wherein the scintillatorcomprises a plurality of scintillator elements, each respective one ofthe plurality of scintillator elements aligned with a different one ofthe plurality of photodiodes.
 15. The x-ray detector of claim 14 whereinthe plurality of photodiodes are positioned in a row, each of theplurality of photodiodes being arranged at a pitch being substantiallyequal to P+G where P is a length of a respective one of the plurality ofphotodiodes in the row and G is a length of a space between adjacentphotodiodes of the plurality of photodiodes in the row and wherein thescintillator comprises and a plurality of scintillator elements arrangedin row and each of the plurality of scintillator elements aligned with adifferent one of the plurality of photodiodes and each of the pluralityof scintillator elements being arranged at a pitch being substantiallyequal to P+G.
 16. The x-ray detector of claim 15 wherein each of theplurality of scintillator elements comprises a width substantially equalto Ps, the width extending in a direction of the row wherein the widthPs is substantially equal to the pitch P+G and each of the plurality ofscintillator elements comprises outer reflective walls surrounding aninner structure of scintillating material and a window positioned in theoutput end the scintillator, the window comprising a width dimension ina direction of the row wherein the width of the window is less thanabout the width Ps of each of the plurality of scintillator elements inthe row direction.
 17. The x-ray detector of claim 16 wherein each ofthe plurality of scintillator elements comprises first and secondsidewalls extending from the input end to the output end of thescintillator, the first and second sidewalls comprising a reflectivesurface and extending toward the second surface at a predeterminedangle.
 18. The x-ray detector of claim 17 wherein the first and secondinterior sidewalls of each of the plurality of scintillator elementsextend toward peripheral edges of the window.
 19. The x-ray detector ofclaim 12, further comprising a printed wiring board connected to each ofthe plurality of electrical contacts the printed wiring boardcomprising: a plurality of semiconductor processor chips connected tothe printed wiring board, each of the plurality of semiconductorprocessing chips comprising: a plurality of amplifiers, each of theplurality of amplifiers connected to a different one of the electricalcontacts and receiving the outputs of a corresponding one of theplurality of photodiodes; analog to digital circuitry connected theplurality of amplifiers; and control logic circuitry multiplexing anoutput of the analog to digital circuitry and outputting multiplexeddigital signals.
 20. A method of fabricating a photodiode detector arrayfor use in an x-ray detector, the method comprising the steps of:forming a layer of intrinsic semiconductor material on a substrate, thelayer of intrinsic semiconductor comprising a first surface and a secondsurface, wherein the first surface is positioned opposite from thesecond surface; providing a first doped layer positioned at the firstsurface, the first doped layer comprising a first conductivity type;providing a plurality of second doped regions positioned at the secondsurface, the second doped region comprising a second conductivity type,the first conductivity type being opposite to the second conductivitytype wherein the plurality of second doped regions detects radiationincident on the first surface and outputs electrical signalscorresponding to the incident radiation; connecting each of a pluralityof electrical contacts to a different one of the plurality of seconddoped regions, the plurality of electrical contacts extending along thesecond surface; locating a first plurality of conductive electrode padson a first board surface of a printed wiring board, each of the firstplurality of conductive electrode pads being aligned with a differentone of the plurality of second doped regions wherein the printed wiringboard is positioned proximate to the second surface; locating a secondplurality of conductive electrode pads on a second board surface of theprinted wiring board, the second board surface located opposite fromfirst board surface, wherein each of the second plurality of conductiveelectrode pads is connected to a different one of the first plurality ofconductive electrode pads; positioning the layer of intrinsicsemiconductor material with the second surface connected to the firstboard surface wherein each of the plurality of electrical contacts beingaligned with a different one of the first plurality of conductiveelectrode pads; and applying a conductive epoxy between each of theplurality of electrical contacts aligned with the first plurality ofconductive electrode pads wherein each of the plurality of electricalcontacts is electrically connected to a different one of the firstplurality of conductive electrode pads.
 21. A method of fabricating anx-ray detector, the method comprising the steps of: providing ascintillator comprising a first outer surface and a second outer surfacewherein the first outer surface is opposite from the second outersurface, the first outer surface receiving radiation from the x-raydetector and light rays being produced at the second outer surface bythe scintillator in response to the radiation received at the firstouter surface; polishing the second outer surface of the scintillator toproduce a substantially planar surface; forming a layer of intrinsicsemiconductor material comprising a first surface and a second surface,wherein the first surface is positioned opposite from the secondsurface; providing a first doped layer positioned at the first surface,the first doped layer comprising a first conductivity type; providing aplurality of second doped regions positioned at the second surface, thesecond doped region comprising a second conductivity type, the firstconductivity type being opposite to the second conductivity type whereinthe plurality of second doped regions detects radiation incident on thefirst surface and outputs electrical signals corresponding to theincident radiation; positioning the layer of intrinsic semiconductormaterial with the first surface is positioned proximate to the secondouter surface of the scintillator; forming a plurality of electrodes onthe second surface of the layer intrinsic semiconductor material, eachof the plurality of electrodes is connected to a different on of theplurality of second doped regions wherein each of the plurality ofelectrodes conducts electrical signals from each of the respective oneof the plurality of electrodes; and connecting each of a plurality ofconnectors to a different on of the plurality of electrodes wherein theplurality of connectors extends along the second surface to convey theelectrical signals to output terminals.
 22. The method of claim 21wherein the scintillator comprises a plurality of scintillator elements,each of the plurality of scintillator elements comprising a respectivebottom wall substantially parallel with the second outer surface of thescintillator wherein the bottom wall comprises a windows at the secondsurface and wherein the step of positioning the layer of intrinsicsemiconductor comprises positioning each of the plurality of seconddoped regions in alignment with a respective window of one of theplurality of scintillator elements.
 23. A scintillator for use with anx-ray detector, the scintillator comprising: a plurality of scintillatorelements, each of the plurality of scintillator elements comprising: acommonly oriented top wall and bottom wall, the top wall and bottom wallhaving a light reflective material disposed thereon; a first sidewalland a second sidewall extending from the top wall to the bottom wall,the first sidewall and the second sidewall having a light reflectivematerial disposed thereon; and a window positioned in the bottom wall,the window having a lateral dimension less than a lateral dimension ofthe bottom wall.
 24. The scintillator of claim 23 wherein portions ofinterior surfaces of the first sidewall and the second sidewall of eachof the plurality of scintillator elements adjacent the respective bottomwall thereof slope inwardly toward the window and extend diagonallybetween adjacent, respective interior surfaces of the first sidewall andbottom wall and between adjacent, respective interior surfaces of thesecond sidewall and the bottom wall.
 25. The scintillator of claim 24wherein: each window comprises a layer of transparent optical material;and the first sidewall and the second sidewall of each of the pluralityof scintillator elements slope inwardly toward, and contact andsurround, a periphery of the respective window, such that a portion ofthe bottom wall contacting and surrounding a window periphery comprisessubstantially a common thickness with that of the window.
 26. Thescintillator of claim 24 wherein the first sidewall and the secondsidewall of each of the plurality of scintillator elements slopeinwardly toward, and define, a perimeter of the window in a plane commonwith an exterior surface of the bottom wall thereof.
 27. Thescintillator of claim 23 wherein each of the plurality of scintillatorelements is arranged at a common pitch.
 28. The scintillator of claim 27wherein a portion of the first sidewall and the second sidewall of eachof the plurality of scintillator elements extends from the firstsidewall and the second sidewall of the respective scintillator elementtoward respective peripheral edges of the window.
 29. The scintillatorof claim 28 wherein portions of the first sidewall and the secondsidewall of each of the plurality of scintillator elements extend fromthe respective first sidewall and the second sidewall of the respectivescintillator element toward the bottom wall of the respectivescintillator element at a predetermined angle.